Method and apparatus for increasing the capacity of a waste heat boiler in a metallurgic smelting furnance

ABSTRACT

The invention relates to a method for increasing the capacity of a waste-heat boiler ( 1 ) processing gases obtained from a metallurgic smelting furnace, such as a flash smelting furnace, and for reducing dust accretions, where liquid is added into the waste-heat boiler ( 1 ) of the smelting furnace. The invention also relates to an apparatus for increasing the capacity of a waste-heat boiler ( 1 ) processing gases obtained from a metallurgic smelting furnace, such as a flash smelting furnace, and for reducing dust accretions, in which case the waste-heat boiler is provided with at least one nozzle ( 2 ) for adding liquid.

The invention relates to the method and apparatus defined in thepreambles of the independent claims for increasing the capacity of awaste-heat boiler processing gases obtained from a metallurgic smeltingfurnace, such as a flash smelting furnace, and for decreasing dustaccretions.

In succession to a metallurgic smelting furnace, the employed waste-heatboiler is generally a so-called tunnel-type boiler operated with adirect gas flow, said boiler being divided into two parts, a radiationchamber and a convection chamber. The purpose of the radiation chamberis to cool the gases so that the molten particles contained in the gasare solidified and the temperature decreases to under the particlesintering temperature before the gases are conducted into the convectionchamber of the waste-heat boiler. In the convection chamber, the finalheat contained by the dust-bearing gases is recovered by a coolingpipework.

When planning a metallurgic smelting process, such as a flash smeltingprocess, it is naturally desirable to minimize investment costs. Owingto the dust content of the exhaust gases, waste-heat boilers connectedto metallurgic smelting may contain dust accretions, which may affectboth the operation of the waste-heat boiler and the smelting processitself. These difficulties may result in possible interruptions in theprocess, which cut the production and become fairly expensive. Theproblems causing dust accretions occur mainly in the following ways:convection cooling packages are blocked in the convection chamber of thewaste-heat boiler, the pipe between the waste-heat boiler and theelectric filter connected thereto is blocked, or accretions are formedon the emission electrodes of the electric filter. In the accretions,there has been found copper sulfate CuSO₄, which means that thesulfatizing of dust is connected to the creation of accretions. When thecopper oxide contained in the dust reacts with oxygen and sulfurdioxide, copper sulfate is created.

Often the limit for the capacity of a smelting furnace is the capacityof the waste-heat boiler connected to the process for sufficientlyreducing the gas temperature, i.e. the capacity of the chamber of thewaste-heat boiler. To increase the capacity of a prior art waste-heatboiler by extending the radiation chamber is a relatively largeinvestment and requires a long interruption in smelting in order to berealized. The temperature before the convection chamber must be setsufficiently low, in order to make the sulfatizing reactions that areimportant for the process to take place exactly in the radiation chamberof the waste-heat boiler. If there is a chance that reactions also takeplace in the convection chamber, the result is that the dust is moreeasily attached to the heat transfer surfaces, and in the worst case thewhole gas line is throttled and finally blocked due to the accretions.

According to the prior art, the described problem is controlled byblowing in the boiler and/or in the uptake shaft so-called sulfatizingair and/or circulation gas, so that the desired reactions take place.According to the patent FI 74,738, the creation of accretions isadjusted by blowing oxygenous gas into the radiation chamber. Thesulfatizing reactions are made to take place earlier, in the radiationchamber, by adding in the radiation chamber some oxygenous gas, such asair. Then the in-blown gas lowers the temperature of the gas containedin the radiation chamber and the reaction area is shifted to theradiation chamber, where the reactions take place in the dust particlesprevailing in the gas phase so that dust accretions are not created.However, by using said method it is not possible to increase the boilercapacity, because the heat amount bound to the circulating gas and airmust be recovered in the convection chamber. Moreover, if too much airis blown in, there is the drawback that when the partial pressure ofoxygen is increased, there is the danger that the sulfur dioxide is inthe reaction converted into sulfur trioxide, thus causing corrosionproblems and an increased creation of unnecessary weak acid in thewashing section of the gas line.

The object of the invention is to introduce a new method and apparatusfor increasing the capacity of a waste-heat boiler in a metallurgicsmelting furnace, such as a flash smelting furnace. In particular, theobject of the invention is to increase the capacity of the waste-heatboiler, so that into the radiation chamber of the waste-heat boiler,advantageously into the first end thereof, there is injected liquid,such as water, whereby the temperature of the radiation chamber—andsimultaneously the reactions taking place in the radiation chamber—arecontrolled. In addition, when necessary, liquid is also added into thesmelting furnace, for instance into the uptake shaft of a flash smeltingfurnace.

The invention is characterized by what is set forth in thecharacterizing parts of the independent claims. The other preferredembodiments of the invention are characterized by what is set forth inthe other claims.

Remarkable advantages are achieved by using the method according to theinvention. According to the invention, in order to increase the capacityof a waste-heat boiler processing the gases obtained from a metallurgicsmelting furnace, such as a flash smelting furnace, an Isasmelt, Ausmeltor Norsmelt reactor, and in order to reduce dust accretions, into thewaste-heat boiler there is added liquid, such as water. The task of theinjected water is to lower the temperature of the gas exhausted from thefurnace, efficiently down to a desired range, so that the heat amountbound by water spraying does not have to be removed completely from thegas in the convection chamber. Part of the liquid can be replaced byair, in which case the task of the oxygen contained in the air is toensure that the sulfatizing reactions take place in the convectionchamber in particular. The capacity of the waste-heat boiler of ametallurgic smelting furnace can be increased by a small investment in agas/liquid/injection arrangement, in addition to which the convectionchamber may need a small extension. The total capacity of the boilergrows in proportion to the heat amount consumed in evaporating the waterand heating the boiler up to the exhaust temperature. Advantageously thesulfatizing reactions in the boiler are made to take place in theradiation chamber, and the gas line is prevented from being blocked.Moreover, the liquid/gas mixture can be injected to the furnace uptakeshaft, from where the gases proceed to the waste-heat boiler. Liquid isadded by placing a required number of nozzles onto the roof or the wallof the radiation chamber of the waste-heat boiler, and possibly also tothe wall of the uptake shaft, or in that part of the smelting reactorfrom where the gases proceed into the waste-heat boiler. The size,orientation angle and position of the nozzle can be adjusted. Throughthe nozzles, liquid is injected either as continuous streams or possiblyso that one of the nozles is out of use. The quantity of liquid iscontrolled according to the incoming gas quantity.

The invention is described in more detail below with reference theappended drawings.

FIG. 1. An apparatus according to a preferred embodiment of theinvention

FIG. 2. A graph of the quantity of water to be added, in proportion tothe quantity of gas.

According to FIG. 1, on the roof of the waste-heat boiler 1 and in theuptake shaft 4 of the flash smelting furnace, there are placed nozzles2. The roof of the waste-heat boiler 1 comprises a hole, through which,via a blowing pipe 3 connected to the nozzle, the liquid/air mixture isinjected to the radiation chamber 5 or the waste-heat boiler 1. Thenozzle 2 is placed in an essentially inclined position with respect tothe gas flow entering from the smelting furnace to the radiation chamber5. The drawing also illustrates the convection chamber of the waste-heatboiler, in which the gases flow from the radiation chamber 5. A similarnozzle can also be used when adding liquid to the uptake shaft 4. Whennecessary, there can be several nozzles, and only one part of them canbe simultaneously in operation.

An example of a preferred embodiment of the method and apparatusaccording to the invention.

EXAMPLE 1

According to the example, the quantity of gas obtained from the flashsmelting furnace is 30 000 Nm³/h, with the following contents: ContentQuantity (Nm³/h) Temperature (° C.) SO₂ (g) 48 14,342 1,350 H₂O (g) 3850 1,350 CO₂ (g) 3 1,012 1,350 O₂ (g) 2 673 1,350 N₂ (g) 44 13,1231,350

Into the waste-heat boiler, there is added 7,000 Nm³/h sulfatizing airwith the temperature of 250 C and the composition of 1,470 Nm³/h O₂ (g)and 5,530 Nm³/h N₂ (g). Let us assume that the gas does not containdust. In addition, let us assume that the temperature of the radiationchamber of the boiler is, in order to secure a perfect operation,lowered down to the temperature 750° C., and that on the other hand, thefinal temperature in the boiler after the convection chamber must notsurpass the temperature 420° C. due to the materials of the nextelectric filter. According to the example, when the capacity of theboiler should be increased, the required quantity of water to beinjected into the boiler and the uptake shaft is illustrated in FIG. 2.In that case, when raising the quantity of gas, the final temperature ofthe radiation chamber of the boiler is still 750° C. Now the increasedheat amount is bound by vaporizing the water and by heating the createdvapor.

For a man skilled in the art, it is apparent that the various preferredembodiments of the invention are not restricted to the above explainedexamples only, but may vary within the scope of the appended claims.

1. A method for increasing the capacity of a waste-heat boilerprocessing gases obtained from a metallurgic smelting furnace, the wasteheat boiler having a radiation chamber and a convection chamber, and forreducing dust accretions, comprising increasing the quantity of gasentering the waste-heat boiler adding liquid into the radiation chamberof the waste-heat boiler of the smelting furnace and controlling thequantity of liquid flow according to the incoming gas quantity so thatthe gas temperature in the waste-heat boiler after the convectionchamber does not surpass 420° C.
 2. A method according to claim 1,wherein liquid is also added into an uptake shaft of the metallurgicsmelting furnace.
 3. A method according to claim 1 wherein part of theliquid is replaced by air.
 4. A method according to claim 1 wherein theliquid is water.
 5. A method according to claims 1, wherein liquid isadded by at least one nozzle.
 6. An apparatus for increasing thecapacity of a waste-heat boiler processing gases obtained from ametallurgic smelting furnace, and for reducing dust accretions, thewaste heat boiler having a radiation chamber and a convection chamber,the waste-heat boiler is provided with at least one nozzle which isarranged to inject liquid into the first end of the radiation chamber ofthe waste-heat boiler and placed in an essentially inclined positionwith respect of the gas flow entering from the smelting furnace to theradiation chamber.
 7. An apparatus according to claim 6 for increasingthe capacity of a waste-heat boiler processing gases obtained from ametallurgic smelting furnace, and for reducing dust accretions, whereinthe wall of the uptake shaft of the smelting furnace is provided with atleast one nozzle for adding liquid.
 8. An apparatus according to claim 6for increasing the capacity of a waste-heat boiler processing gasesobtained from a metallurgic smelting furnace, and for reducing dustaccretions, wherein the nozzles are arranged for increasing liquid, partof which liquid is replaced by air.
 9. An apparatus according to claim6, for increasing the capacity of a waste-heat boiler processing gasesobtained from a metallurgic smelting furnace, and for reducing dustaccretions, wherein the nozzles are arranged for adding liquid, saidliquid being water.